The present invention relates in general to a remote vehicle locating system for assisting a user to find their vehicle, such as in a parking lot, and, more specifically, to a vehicle locating system having receivers of GPS satellite signals mounted in a personal fob and in the vehicle to be located.
Car locating systems having a portable GPS receiver carried by the vehicle user and a car-mounted GPS receiver have been proposed. Typically, a GPS-derived location of the car when it is parked is downloaded to the portable receiver as a waypoint. Later, when the user wants to return to the vehicle, the portable receiver is actuated to determine its current location and to then provide a direction and distance to the vehicle waypoint.
GPS receivers depend on several types of information contained within the signals broadcast by the GPS satellites in order to make a determination of their geographic position. A 1 MHz pseudo-random noise code modulates a 1575.42 MHz carrier, with each GPS satellite having a unique pseudo-random noise code. The carrier is also modulated with a 50 Hz navigation message containing data describing satellite orbits (i.e., ephemeris), clock corrections, and other GPS system data.
To determine the range to a particular satellite, the receiver determines a time difference between when a pseudo-random noise code is admitted from the satellite and when it is received by the receiver (with the final receiver position being determined by triangulation of ranges to multiple satellites). An accurate determination of the time difference depends greatly on the data in the navigation message. However, due to the low data rate used for the navigation message, it takes 12.5 minutes to send a complete message.
GPS receivers typically maintain baseline navigation data, such as satellite ephemeris data, and refine the data based on updated information received in the navigation message. Accuracy of the clock utilized in a GPS receiver also depends on continuous monitoring of both the navigation data and the pseudo-random noise codes from multiple satellites.
When a GPS receiver is activated after having been powered off (i.e., a cold start), it does not possess any current navigation data and its internal clock may be significantly off from GPS time. Therefore, any position determinations are subject to greater inaccuracy until sufficient amounts of navigation data and monitoring of multiple satellite signals have been achieved.
A handheld mobile GPS receiver is typically powered by a battery. A miniaturized receiver such as one contained in a vehicle remote entry fob can only accommodate a small battery. Because of the limited capacity of a small battery, it is undesirable to run such a GPS receiver continuously since the battery life would be consumed too quickly. Upon a cold start, however, a sufficiently accurate position determination may not be possible until after an unacceptably long delay.